Solid polymer fuel cells having a polymer electrolyte membrane can be easily reduced in size and weight, and are thus expected to be put into practical use as a power source in moving vehicles such as electric automobiles, and compact cogeneration systems. However, solid polymer fuel cells have a comparatively low operating temperature, which makes it difficult to effectively utilize their exhaust heat for auxiliary power and the like. Thus, for practical use of such fuel cells, there is a need for performance capable of obtaining high power generation efficiency and high output density under operating conditions of a high anode reaction gas (pure hydrogen etc.) utilization rate and a high cathode reaction gas (air etc.) utilization rate.
The electrode reactions at the respective catalyst layers of the anode and the cathode of a solid polymer fuel cell proceed at a three-phase boundary (hereinafter, referred to as “reaction site”) where the respective reaction gas, catalyst, and fluorinated ion exchange resin (electrolyte) are simultaneously present. Thus, each electrode reaction only proceeds at the three-phase boundary where the gas (hydrogen or oxygen), which is the active material, protons (H+), and electrons (e−) can be simultaneously exchanged among them.
An example of an electrode having such a function is a solid polymer electrolyte-catalyst composite electrode including a solid polymer electrolyte, carbon particles, and a catalyst material. This electrode may be, for example, a porous electrode containing carbon particles supporting the catalyst material and a solid polymer electrolyte which are three-dimensionally distributed in admixture and having a plurality of pores formed thereinside. The carbon supporting the catalyst forms an electron-conductive channel, the solid electrolyte forms a proton-conductive channel, and the pores form a channel for the supply and discharge of oxygen, or the hydrogen and water which is generated as a product. These three channels are three-dimensionally spread throughout the electrode, so that numerous three-phase boundaries which allow the simultaneous exchange of gas, protons (H+), and electrons (e−) are formed, providing electrode reaction sites.
Thus, in a solid polymer fuel cell, a catalyst such as a metal catalyst or a metal-loaded catalyst (e.g., metal-loaded carbon with a metal catalyst such as platinum or the like on a carbon black carrier having a large specific surface area) has been coated with a fluorinated ion exchange resin which is the same as or different from the polymer electrolyte membrane to form a constituent material of the catalyst layer, which has been used to increase the number of reaction sites in the catalyst layer through so-called three-dimensional arrangement of the reaction sites and thereby to utilize more efficiently the expensive precious metal such as platinum, a catalyst metal.
Conventionally, an electrode catalyst on which catalyst metal fine particles of platinum or platinum alloy, which is the active catalyst material, are supported in a highly dispersed state on a conductive carbon carrier having a large specific surface area, such as carbon black, is used for both the anode and the cathode. Supporting fine particles of the catalyst metal in a highly dispersed state increases the electrode reaction area, which increases catalyst performance.
However, because the electrode reaction at the cathode has a large activation energy, an overvoltage is generated across the cathode. As a result, if the cathode is exposed to a noble potential environment of about 1.2 V, the carbon carrier is lost through corrosion to liberate and agglomerate platinum, which has caused the problem that the cell life decreases.
Accordingly, an electrode catalyst was thought of which was produced using a heat-treated carbon powder for the carrier. It is known that carbon powder generally has a structure similar to that of graphite if heat treated at a high temperature of 1,000 or more, whereby corrosion resistance is improved.
As for graphite, it is a hexagonal crystalline substance of carbon and consists of crystallites in a layered structure in which planes of highly developed fused carbon rings lie on one another. Although the carbons in the plane of each layer are linked by strong covalent bonds (sp2 bonds), the respective layers are bonded by weak van der Waals forces. The face on which the edge face of such a carbon fused ring is oriented is called the “edge face”, and the face on which the carbon fused ring plane is oriented is called the “basal face”.
However, although heat treated carbon powder has improved corrosion resistance, its specific surface area decreases, meaning that platinum cannot be supported in a highly dispersed state. Therefore, an electrode catalyst using a carrier having a graphite structure has the drawback that its activity is lower since it is formed only of a heat treated carbon powder.
Further, since graphite has a developed layer structure, graphite does not easily support the catalyst metal, so that the supported catalyst metal tends to peel off. Thus, in some cases the catalyst component peels off over time through use, and catalyst performance deteriorates. In addition, for a conventional method, catalyst metal particles are more close to one another, and larger particles tend to form by sintering of the catalyst metal.
In view of this, the inventors of Japanese Patent Publication (Kokai) No. 2005-216772 discovered that the basal face and the edge face present in a graphitized carbon have different characteristics; specifically, that although the basal face is energetically stable, the edge face is energetically unstable and is activated. The inventors of that document also discovered that since the catalyst metal tends to be supported more on the edge face, by increasing the number of edge faces, the effective surface area of the carrier can be enlarged, and the catalyst metal utilization rate can be improved. As a result, because the power output generated per unit cell area increases, the cell can be reduced in size, or the requirement of the catalyst or precious metal can be reduced, which contributes to cost reduction.
Such a technique for improving the durability of a supported catalyst by graphitizing a carrier loaded with a catalyst is known. Further, carbon particles as a carrier have been reported in which the average lattice spacing d002 of the [002] face, the dimension Lc (002) in the thickness direction of the crystallites, and the specific surface area are defined in specific ranges, because they provide better water repellency and corrosion resistance for electrode catalyst layers. For example, it is an object of Japanese Patent Publication (Kokai) No. 2001-357857 to obtain a cathode which has high activity in the reduction reaction of oxygen, and excellent water repellency and corrosion resistance, and a solid polymer fuel cell which has excellent output characteristics and driving stability as a result of having such cathode. This document discloses an electrode catalyst in which platinum or a platinum alloy is supported on a carbon carrier having an average lattice spacing d002 of 0.340 to 0.362 nm, a crystallite dimension Lc of 0.6 to 4 nm, and a specific surface area of 260 to 800 m2/g.